The present invention pertains to a yellow ink for inkjet printing and, in particular, to a yellow ink comprising a combination of specific yellow colorants. The present invention further pertains to an ink set comprising this yellow ink.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
An ink jet ink set for color printing will generally comprise a cyan, magenta and yellow (CMY) ink, which are referred to as the primary colors. An ink set will also commonly comprise a black ink (CMYK).
A suitable ink should generally exhibit good crusting resistance, good stability, proper viscosity, proper surface tension, good color-to-color bleed alleviation, rapid dry time, consumer-safety and low strike-through.
In addition, the ink set should provide printed images having good color characteristics, such as correct hue and high chroma. Preferably, the ink set will achieve these favorable characteristics on a range of media including plain paper as well as specialty media such as transparency film and coated paper. Also, preferably, the hard copy output is reasonably lightfast.
While some of these conditions may be met by ink vehicle design, other conditions must be met by the proper selection and combination of the colorants. The selection of the colorants becomes especially important when additional limitations are placed on the choice of the colorants because of other system requirements, such as the color-to-color bleed control mechanism.
The term “bleed” means the invasion of one color into another, once the ink is deposited on the print medium. It can be seen as a ragged border between two adjacent colors. The occurrence of bleed is especially problematic between a black ink and color ink because it is all the more visible. Preferably bleed is minimized or eliminated so that borders between colors are clean and sharp.
U.S. Pat. No. 5,488,402 discloses a method for preventing color bleed between two different color ink compositions wherein the first ink is anionic and comprises a coloring agent which includes one or more carboxyl and/or carboxylate groups, and the second ink includes a precipitating agent which is designed to ionically crosslink with the coloring agent in the first ink to form a solid precipitate in order to prevent bleed between the two ink compositions. Multivalent metal salts are disclosed as being useful as the precipitating agent.
U.S. Pat. No. 5,518,534 discloses an ink set for alleviating bleed in multicolor printed elements employing a first ink and a second ink, each containing an aqueous carrier medium and a colorant; the colorant in the first ink being a pigment dispersion and the second ink containing a salt of an organic acid or mineral acid having a solubility of at least 10 parts in 100 parts of water at 25° C., wherein the salt is present in an amount effective to alleviate bleed between the first and second inks.
To take advantage of a bleed control mechanism involving salts, it is necessary to have a set of inks that can provide suitable performance characteristics while maintaining reliability in the presence of those salts. U.S. Pat. No. 6,053,969, for example, discloses an ink set with salt compatibility that addresses these needs. A key aspect of this art is the selection of yellow colorant for the yellow ink in this ink set. The difficulties of selecting the yellow colorant are described. For example, DY132 has favorable hue angle, chroma and lightfastness, but is incompatible with precipitating agents (inorganic salts). DY86 has favorable, chroma and lightfastness, but a lower than desired hue angle and no compatibility with precipitating agents. AY23 has favorable hue angle, chroma and compatibility with precipitating agents, but poor lightfastness. AY17 has favorable chroma, lightfastness and compatibility with precipitating agents, but higher than desired hue angle. IIford Y104 (CAS Number 187674-70-0) has favorable chroma, lightfastness and compatibility with precipitating agents, but lower than desired hue angle. However, a yellow ink with a combination of yellow colorants, namely AY17 and IIford Y104 achieves all the target attributes, namely a hue angle of 90-95, plain paper chroma of at least 70, good lightfastness and compatibility with inorganic salts.
Co-owned and co-pending application U.S. application Ser. No. 11/472,710 discloses other yellow colorant combinations, namely Acid Yellow 17 with Acid Orange 33 and/or Reactive Yellow 181, that achieve target attributes similar to the AY17/IIford Y104 combination in U.S. Pat. No. 6,053,969.
However, the aforementioned salt stable yellow inks, and ink sets containing same, have some performance deficiencies on specialty paper. In particular, the regions of the CYM composite black (combination of cyan, magenta and yellow inks) printed on photo quality microporous glossy paper have undesirable haze and hue shift.
A need still exists for improved inks, particularly yellow inks, and ink sets that provide appropriate color, lightfastness and bleed control on both plain paper and specialty paper, such as photoglossy paper, and that exhibit little or no haze or hue shift.